docs: def two_phase_dP_dz_gravitational(angle, alpha, rhol, rhog, g=g):
      r'''This function handles calculation of two-phase liquid-gas pressure drop
      due to gravitation for flow inside channels. This is a differential 
      calculation for a segment with an infinitesimal difference in elevation for
      use in performing integration over a pipe as shown in [1]_ and [2]_.
      
      .. math::
          -\left(\frac{dP}{dz} \right)_{grav} =  [\alpha\rho_g + (1-\alpha)
          \rho_l]g \sin \theta
          
      Parameters
      ----------
      angle : float
          The angle of the pipe with respect to the horizontal, [degrees]
      alpha : float
          Void fraction (area of gas / total area of channel), [-]
      rhol : float
          Liquid phase density, [kg/m^3]
      rhog : float
          Gas phase density, [kg/m^3]
      g : float, optional
          Acceleration due to gravity, [m/s^2]
  
      Returns
      -------
      dP_dz : float
          Gravitational component of pressure drop for two-phase flow, [Pa/m]
          
      Notes
      -----
          
      Examples
      --------    
      >>> two_phase_dP_dz_gravitational(angle=90, alpha=0.9685, rhol=1518, 
      ... rhog=2.6)
      493.6187084149995
  
      References
      ----------
      .. [1] Rohsenow, Warren and James Hartnett and Young Cho. Handbook of Heat
         Transfer, 3E. New York: McGraw-Hill, 1998.
      .. [2] Kim, Sung-Min, and Issam Mudawar. "Review of Databases and
         Predictive Methods for Pressure Drop in Adiabatic, Condensing and
         Boiling Mini/Micro-Channel Flows." International Journal of Heat and
         Mass Transfer 77 (October 2014): 74-97.
         doi:10.1016/j.ijheatmasstransfer.2014.04.035.
      '''
      angle = radians(angle)
      return g*sin(angle)*(alpha*rhog + (1. - alpha)*rhol)